Optical communications systems encode information onto a light beam at a transmitting location, transmit the light beam through free space or a medium such as an optical fiber, and then decode the information from the light beam at a receiving location. A great deal of information may be encoded onto the light beam due to its high frequency. Additional information may be transmitted by encoding it onto a second light beam having a slightly different wavelength than the first light beam, mixing the two light beams together at the transmitting location (or several different transmitting locations), transmitting the mixed light beam, separating the two light beams at the receiving location (or several different receiving locations), and then decoding the two sets of information from the two light beams. The amount of information that may be transmitted is increased yet further by using additional light beams in a similar manner, with all of the light beams at slightly different wavelengths.
To implement such an optical communications system, the two or more light beams having slightly different wavelengths must be mixed together (i.e., combined into a single beam for transmission), a process termed wavelength-division multiplexing, and later separated apart from the single transmitted beam, a process termed wavelength-division demultiplexing. The mixing and separating operations are reciprocal, so that the same type of apparatus may be used in a reciprocal manner, to perform both operations. The apparatus used to perform the multiplexing and demultiplexing is termed a multiplexer/demultiplexer apparatus, which may be shortened to a “mux/demux apparatus.”
One well-established mux/demux apparatus accomplishes the mixing and separation with thin-film optical filters. When a light beam is incident upon the thin-film optical filter at a precisely defined angle of incidence, the thin-film optical filter passes light within a very narrow spectral pass band characteristic of the thin-film optical filter, and reflects all other wavelengths of light. This property is used to mix light beams together or to separate them, as required in the mux/demux apparatus. Many variations of mux/demux apparatus use the thin-film optical filters in the light communications systems.
However, the cost of such systems is substantially greater than desired, due to the low manufacturing yield of suitable thin-film optical filters. The thin-film optical filters are made by depositing a sequence of typically over one hundred, and sometimes as many as several hundred, very thin layers onto a substrate in a pattern and thickness that, taken together, produces the desired pass band characteristic. In a typical manufacturing operation, the sequence of layers is deposited onto a large substrate, which is then diced to produce a number of the individual thin-film optical filters. It is often found that, with manufacturing variations in this complex processing, only a small fraction of the final thin-film optical filters satisfy the specifications that must be met for the mux/demux apparatus to be functional as designed, when the mux/demux apparatus is assembled using conventional techniques.
There is therefore a need for an improved approach to mux/demux apparatus and assembly approach that results in a satisfactory sequence of mixing or separating the light beams, at a lower cost than presently possible. The present invention fulfills this need, and further provides related advantages.